This invention relates to image storage of a changeable display, particularly in a window system. In a window system, several windows are displayed on a screen of display, stacked(or overlapped) to each other. In each window, a picture is displayed, and a part of the picture overlapped by other windows, is not displayed on the screen. Sequence of windows in the stack is changed rather frequently, and a window in a lower part of the stack will come at the top of the stack where the whole picture of the window must be displayed on the screen.
For such a quick change of a display, a BitBLT (Bit Block Transfer) hardware is provided for transferring a block data from one area of memory to another in a very short time.
As for a quick change of a character display, font images of characters are also stored in the VRAM, and a font image of a character is transferred to a desired position in the VRAM by the BitBLT hardware through specifying the identifier of the font image.
But when another set of a stack of windows is to be displayed on the screen, the picture images stored in the VRAM for storing displayed images must first be transferred to an area of a main memory reserved as RAM for storing saved images. After the images of current display are saved in the RAM for storing saved images, a set of new images is transferred from the RAM to the VRAM for storing displayed images.
For the transfer between the VRAM for storing displayed images and the RAM in the main memory for storing saved images, a system bus is used, and a fairly longer time is required for signal transmission compared with that by the BitBTL hardware. Therefore, in a heretofore known image storage of a window system, it has taken a fairly long time to display saved images once more on the screen.